1. Field of the Invention
The present invention relates to a rubber composition improved in an anti-static property and a pneumatic tire using the rubber composition.
2. Description of the Related Art
Natural rubber and synthetic rubber are used in various industrial fields including tires. In general, these rubbers are insulating materials and substantially have a property to be liable to carry static charge.
It has so far been carried out as a publicly known technique to reduce a volume resistance of rubber and improve the anti-static property by blending a rubber composition with carbon black. However, this technique is restricted to black color products.
It is known that carbon black blended into a rubber composition increases hysteresis loss even in black color products such as a tire. In recent years, it is investigated to decrease the blending amount thereof or to substitute a white filler such as silica for carbon black in low fuel consumption tires. In this case, conductivity provided by carbon black is reduced, which in turn causes a result that static charge is liable to be carried.
If tires carry static charge, concerned are the inconveniences, for example, that when passengers get off a car they have an unpleasant feeling because of an electric shock, and that radio waves are generated when static charge accumulated in tires is discharged, so as to cause radio noises.
A technique of blending nonionic surfactants or phosphoric acid esters is known as a conventional anti-static technique for a rubber composition for tires, as disclosed in International Patent Application Laid-open No. WO 95/31888 applied by one of the present inventors. This technique is excellent in terms of being capable of reducing a volume resistance of a rubber composition but has a problem that an anti-static effect is reduced in a relatively short period of time depending on conditions of using the tires.
On the other hand, anionic anti-static agents are already publicly known as anti-static agents for plastics, but it is not known that they provide rubber compositions capable of maintaining an anti-static performance over a long period of time in combination of rubber including diene base rubber with a specific filler.
Although it is known that polyoxyalkylene glycol esters are excellent as a low temperature resistant and heat resistant plasticizer for butadiene rubber as described in Japanese Patent Publication No. Hei 6-4722 applied by one of the present inventors, it is not yet known that they have an anti-static performance.